The present invention pertains to optical films that are useful in colored displays. Such displays are frequently used as a means to display information in an eye-catching manner, or to draw attention to a specific article on display or for sale. These displays are often used in signage (e.g., outdoor billboards and street signs), in kiosks, and on a wide variety of packaging materials.
It is particularly advantageous if a display can be made to change color as a function of viewing angle. Such displays, known as “color shifting displays”, are noticeable even when viewed peripherally, and serve to direct the viewer's attention to the object on display.
In the past, color has usually been imparted to displays by absorbing inks which are printed onto card stock or onto a transparent or translucent substrate. However, such inks are typically not color shifting (i.e., the colors of such inks do not normally change as a function of viewing angle).
Some color shifting inks have also been developed, chiefly for use in security applications. However, in addition to their considerable expense, some inks of this type are opaque and are therefore not suitable for backlit applications. Furthermore, such inks are typically based on multilayer stacks of isotropic materials, and hence lose color saturation as viewing angle increases.
Color shifting pigments are also known. For example, a family of light interference pigments are commercially available from Flex Products, Inc. under the trade name CHROMAFLAIR®, and these pigments have been used to make decals. The product literature accompanying these decals describes them as consisting of color shifting pigments in a commercial paint formulation, which is then applied to a vinyl substrate. However, the color shifting effect provided by these materials is only observable at fairly large oblique angles, and is limited to a shift between two colors. Also, these materials, which are apparently described in U.S. Pat. No. 5,084,351 (Phillips et al.), U.S. Pat. No. 5,569,535 (Phillips et al.), and U.S. Pat. No. 5,570,847 (Phillips et al.), all assigned to Flex Products, exhibit fairly low color intensity (see, e.g., FIGS. 7–9 of U.S. Pat. No. 5,084,351). Similar materials are described in U.S. Pat. No. 5,437,931 (Tsai et al.).
An iridescent plastic film is currently sold under the trade name BLACK MAGIC™ by the Engelhard Corporation. The film has been advertised in Cosmetics & Personal Care Magazine (September-October 1997) as a black tinted, translucent film 0.7 mil thick but containing more than 100 layers which provides an effect similar to that seen with neon tetra fish, peacock feathers and oil films. The plastic film is a multilayer stack of optically thin films. Thickness variations in the films results in color variations across the area of the film. Although the deviations of the thickness caliper from its average value are not large, they are significant in terms of the color differences in adjacent areas. The various versions of the film are not labeled as a single reflectance color, but instead as dual colored films. For example, the film is commercially available in blue/green and red/green color combinations, among others.
Other color shifting films have also been developed. Some such films are based on multilayer films of metals, metal salts, or other inorganic materials. Thus, U.S. Pat. No. 4,735,869 (Morita) describes titanium dioxide multilayer films which exhibits various combinations of reflection and transmission colors (e.g., green reflection with magenta colored transmission).
Other multilayer color shifting films are known which are polymeric. Thus, U.S. Pat. No. 5,122,905 (Wheatley et al.), in describing the films of U.S. Pat. No. 3,711,176 (Alfrey, Jr. et al.), notes that the color reflected by those films is dependent on the angle of incidence of light impinging on the film. However, these films are not well suited to color displays, since the color shift observed in these films is very gradual and the color saturation is very poor, particularly at acute angles. There is thus a need in the art for a color shifting film useful in display applications which exhibits sharp color shifts as a function of viewing angle, and which maintains a high degree of color saturation. There is also a need in the art for uniformly colored polymeric interference filters.
Various birefringent optical films have been produced using strain hardening (e.g., semicrystalline or crystalline) materials. These materials have proven advantageous in the production of multilayer optical films, since desired matches and mismatches in the refractive indices of these materials can be achieved through orientation. Such films are described, for example, in WO 96/19347.
There is also a need in the art for a polymeric multilayer optical film having good color uniformity. Multilayer films made from extruded polymeric materials have been found to be highly susceptible to distortions in layer thickness and optical caliper, which result in color variations and impurities across the width of the film. This problem was commented on in Optical Document Security, 251–252 (Ed. R. van Renesse, 1994). In describing the multilayer polymeric films produced to date by Dow Chemical Company and their licensee, Mearl Corporation, the reference notes that control of thickness variations of the individual layers in these films is very difficult and that, as a result, the films exhibit “countless narrow streaks of varying color, few of which are wider than 2–3 mm.” Id. At 251. This problem was also noted in Dow's patent U.S. Pat. No. 5,217,794 (Schrenk) at Col. 11, Lines 19–32, where it is noted that the processes used to make the films described therein can result in layer thickness variations of 300% or more. At Col. 10, Lines 17–28, the reference notes that it is characteristic of multilayer polymeric bodies having optically thin layers (i.e., layers whose optical thickness is less than about 0.7 micrometers) to exhibit nonuniform streaks and spots of color. A similar comment is made at Col. 2, Lines 18–21, with respect to the films of U.S. Pat. No. 3,711,176 (Alfrey, Jr. et al.). As demonstrated by these references, there is a long-standing need in the art for polymeric multilayer optical films (and a method for making the same) which have high color uniformity.
Other polymeric multilayer optical films are known which rely on optically thick or optically very thin layers for their primary reflection band. Such films avoid some of the iridescence problems encountered with other multilayer polymeric films, primarily because the bands of iridescence are too close to be discerned by the human eye. However, since the reflection of visible light is provided by higher order harmonics of primary reflection bands located in the infrared region of the spectrum, the ability of the films to produce high reflectivities of visible light is compromised. There is also a need in the art for multilayer polymeric optical films (and a method for making the same) whose primary reflection bands arise from optically thin layers (e.g., layers having an optical thickness between 0.01 micrometers and 0.45 micrometers) and which exhibit highly uniform color.
These and other needs are met by the color shifting films of the present invention, as hereinafter described.